Eerie Cosmic-Ray ‘Cavity’ Found Lying on Earth’s Moon

The ever-present hail of cosmic rays streaming into the Solar System from an outer galaxy may not be what we thought.

According to Chang’e 4 on the far side of the Moon, there is a mysterious ‘bag’ of cosmic ray flux between the Earth and the Moon that occurs when the two bodies line up in the right direction.

It is a discovery that suggests that cosmic rays are not distributed as evenly as we thought, perhaps opening up opportunities to explore the atmosphere that can help reduce the risk of radiation produced by these particles.

Space can be a busy place, alive with all kinds of unusual hijinks that sprinkle the universe with energetic particles – such as supernova explosions and supernova remnants that shoot cosmic rays out of control at high speeds. These are mostly protons, some helium nuclei, and a small number of heavier atomic nuclei, and are thought to be relatively ubiquitous.

It’s also ionizing radiation – you know, the stuff that can knock electrons off the atoms in your body, damage your DNA, and increase your risk of mutations that can give you cancer – so, not a good time.

Galactic cosmic rays (GCRs) are heavily absorbed by Earth’s atmosphere before reaching the surface. However, they pose a significant radiation hazard to astronauts and high-altitude pilots, which is accepted as part of the job and taken into account when designing the projects and technologies that support them.

The flow of the GCR – that is, the strength of the GCR core – can change depending on what the Sun is doing. It drops significantly during the solar maximum because the increasing solar wind and magnetic activity deflect a large fraction of the particles.

The Sun is not the only source that can block GCRs, according to a new analysis from an international team, the Earth’s gravity can, too – but the Sun is still indirectly involved.

The observation comes from Chang’e 4, which is located on the far side of the Moon using its Lunar Lander Neutron and Dosimetry (LND) instrument to observe protons from 2019. It can only do this during the lunar day, when its area is illuminated by the Sun, since the Moon is too cold for a man who lives on the ground to work after dark.

But this daily activity is a good opportunity to measure the effect of the Earth’s gravity on the flow of the GCR. The researchers collected data from 31 lunar cycles and looked for changes in proton flow as the Moon moved around the Earth.

They found that, in one part of its cycle – the pre-noon part, before it reaches midday relative to the Sun – the Moon encounters an area where the proton flux is 20 percent lower than in the other part of the cycle.

Researchers believe this may be related to interplanetary gravity, which is the part of the Sun’s gravity that reaches far into the Solar System.

As the Sun rotates, its gravitational pull twists into a spiral known as the Parker spiral, and when this aligns with the Earth-Moon system in just the right direction, the GCR hole breaks open.

“In general, the motion of the charged particles is characterized by a helical spiral along the lines of gravity,” the researchers write.

“When the Moon is in the prenoon region under Parker spiral conditions, the IMF field lines can adjust in such a way that they connect the Moon to the strong magnetic field of the Earth. Therefore, the movement of particles along those field lines, especially the protons that we record here, are affected by the strong magnetic field of the Earth.”

So the curved lines of gravity around the region are, at some point, inclined to Earth and interact with the planet’s gravity, creating a “shadow” type of GCR. As the Moon passes through the shadow, a process that takes about two days, Chang’e 4 records the dip in proton flux from the GCRs.

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It’s a discovery, researchers say, that could provide a way to reduce exposure to solar radiation.

“This finding provides a potential strategy for strategic planning, especially for [crewed] monthly activities, as activity can be timed to coincide with these low radiation periods to reduce the risk of exposure,” the researchers write.

“Future studies with expanded datasets may further elucidate the spatial extent and behavior of this region, providing deeper insights into radiation protection strategies, not only for the Earth-Moon system but potentially for missions near other powerful bodies in the Solar System.”

The findings were published in Advances in Science.